Alternatives to Petroleum Based Aviation Fuel

The use of petroleum based fuels has been the preferred means of powering aircraft in the aviation industry since the concept of heavier than air flight became a reality. Petroleum products have been readily available, easy to handle, offer excellent performance, and have been the most economical. Today’s environment however, has the aviation industry searching for alternative fuel sources. Concerns of energy independence, the exorbitant increase in fuel prices and growing environmental concerns have invigorated the interest of alternative energy sources. The civil aviation industry and the aviation arms of the military have escalated research in the area of alternative jet fuels in order to meet the challenges presented today.

The use of alternative jet fuels is not a new concept. Early aviation researchers studied various forms of energy throughout the development of aviation engine and aircraft design. Global instability in the oil industry during the 1970s and early 1980s challenged the permanence of availability and heightened awareness of vulnerability to foreign entities. But the economics of overabundant supplies and low costs of the late 1980s through the 1990s diminished the apparent necessity for research.

The current soaring cost in jet fuel has rejuvenated the recognition of essential research in the field of alternative jet fuel. Researchers are studying the feasibility of various types of alternatives. Natural gas, shale oil, and coal are among the fossil fuels being evaluated as synthetic fuels derived through the Fischer-Tropsch process. There is research in the biofuel products of ethanol and biodiesel and the cryogenic fuels made from liquid hydrogen and liquid methane. Aircraft designers are addressing challenges to fuel storage issues, engine performance, system design changes and certification issues. Environmental concerns are also being investigated to ensure lower emissions and the practicality of available arable land. Research which began many years ago has been recognized as necessary today to address the growing need for alternatives to petroleum based energy, specifically petroleum based jet fuel.

Historical Development

Petroleum based jet fuel is overwhelmingly the more accepted form of energy producing fuel for jet engines. But, the recent thrust for the use of alternative fuels is not a new concept for jet engine designers. Early proponents of the jet engine claimed that “these new engines could operate on any fuel from whiskey to peanut butter” (Air BP). In fact, the first jet engine, designed by Hans von Ohain in 1935 was powered by hydrogen. Von Ohain reported: “The apparatus fully met expectations. It reached the anticipated performance, it handled well in acceleration and deceleration, probably due to…the great stability of the hydrogen combustion over the wide operational range” (Pioneers). Further early jet engines were developed that used hydrogen but the need for a fuel that had higher energy content per weight and volume led to the adoption of kerosene as the standard aviation fuel (ICAO, 2007).

During the 1950s and 1960s research continued for alternatives to petroleum based fuels or “conventional jet fuel”. Liquid hydrogen and other cryogenic fuels were studied as were exotic fuels such as boron compounds. The energy crisis of the 1970s and concerns for stability in oil prices increased the need for finding alternative jet fuels. Much research concentrated on biomass conversion to fuel including vegetable oil or biokerosene used to power Embraer turbo prop aircraft in Brazil (ICAO). In South Africa, the embargo to end apartheid motivated research into semi-synthetic aviation fuel known as Sasol, a mixture of petroleum derived and synthetic kerosene. In the latter part of the 1980s, oil flooded the market place. Many programs to research alternative fuels were abandoned due to the low cost of conventional jet fuel and are just recently being reconsidered.

Concerns of rising fuel costs, energy supply security and the environmental effects of aviation are providing a significant stimulus to take a fresh look at the use of alternative fuels for aviation (ICAO). Although the airline industry has immensely improved their efficiency in fuel consumption, there is concern that oil production may not be capable of keeping pace with demand. These concerns have researchers again looking to development of alternatives to conventional jet fuel. Daggett and Hadaller contend “It is essential that alternatives to crude oil be developed to help stabilize energy supplies and their associated prices and to address global warming issues” (NASA). Researchers are analyzing the viability of several types of alternative fuels. Fossil fuels, Fischer-Tropsch (FT) synthetic fuels, biofuels and cryogenic fuels are being evaluated in an attempt to find viable alternatives. Some alternatives, particularly FT process synthetic fuels show much potential. U.S. engine manufacturers are currently pursuing efforts to expand the current certification of blended Sasol/petroleum fuel to qualify pure Sasol for operational use (ICAO). The available alternatives, some with more immediate potential than others, have some challenges to be met. But each could serve to meet some portion of future energy demand.

Fischer-Tropsch Synthetic Fuel

The Fischer-Tropsch (FT) process was discovered in 1923 by Franz Fischer and Hans Tropsch, German researchers working at the Kaiser Wilhelm Institute searching for an alternative to petroleum based fuel during World War II. Initially, the process was incorporated to convert coal to liquid fuel in order to capitalize on Germany’s rich coal resource. The FT synthesis converts a mixture of carbon monoxide and hydrogen, called synthesis gas, into higher molecular weight hydrocarbons (Chevron). The synthesis begins with carbon monoxide hence; any source of carbon could be used for the process. As mentioned previously, the first plants were designed to utilize coal as the feedstock. In using a conversion method known as coal-to-liquid (CTL), coal is converted into synthetic fuel. Increase interest in natural gas as the feedstock has many researchers incorporating gas-to-liquid (GTL) conversion and biomass-to-liquid (BTL) is showing promise as a feedstock. During the gasification step, the connection to the starting material is lost, so FT liquids produced from any starting material will be essentially the same (Chevron). The instability of oil supply, higher costs for supplied oil, and increased environmental concerns has interest in alternatives offered by the FT process at a new peak.

Coal to Liquid

Coal reserves are abundant throughout the world and could be utilized as feedstock for synthetic fuel. There are currently two methods of converting coal for use as transportation fuel. One method, which has much opposition from environmentalists, is the direct liquefaction process. The more favored technique is through FT synthesis. The Sasol Company of South Africa has developed the FT process to a point of commercialization of CTL synthetic fuels and has gained approval for blends of 50 percent by volume synthetic fuel (Chevron). As of April 2008, Sasol has gained approval for 100% synthetic CTL jet fuel produced at Sasol’s Synfuels facility in Secunda, South Africa (NextEnergy). The coal is mined and crushed before being converted to carbon monoxide and Hydrogen gases. These gases are purged and adjusted to proper mixes before entering the FT synthesis unit. This process does require extensive amounts of energy to produce the fuel and can result in large amounts of carbon dioxide (CO2) being released into the atmosphere. The process can only be considered a long-term, viable alternative to petroleum if the CO2 emissions can be captured and permanently sequestered (NASA). Research continues to resolve the issue of seizing the CO2 emissions and permanently containing them. The promising off-set is that synthetic fuels contain no sulfur, and if not blended, zero aromatic components. The fuel produces less particulate matter and because synthetic fuels have higher hydrogen content, they may offer some reductions in CO2 emissions (ICAO) when burned as fuel.

Gas to Liquids

Gas to liquids (GTL) synthetic fuel is currently experiencing growing support. GTL, another FT synthetic fuel which uses natural gas as the feeder stock is nearly an identical replacement for kerosene (ICAO). The United States has reenergized the development of synthetic fuels with the Total Energy Development (TED) program (NASA). In 2006, Syntroleum Corporation, a Tulsa, Oklahoma based synthetic fuel provider, supplied 100,000 gallons of synthetic Fischer-Tropsch (FT) jet fuel to the Department of Defense (DOD), which used the fuel in a 50/50 blend with conventional jet fuel in several test flights of a B-52 bomber (Syntroleum). August, 19, 2008 The United States Air Force (USAF) successfully flew an F-15E Strike Eagle using GTL synthetic fuel. The test was part of the USAF program to certify the entire Air Force fleet on synthetic fuel by 2011 (Creel). The Air Force’s B52 Stratofortress is already certified to use the synthetic fuel and the results of the F-15 flights are expected to permit certification of that fleet. The Air Force is currently in the process of certifying the B-1B Lancer, C-17 Globemaster III, KC-135 Stratotanker, and the F-22 Raptor (Creel).

In the civilian market, an Airbus A380 flew a three-hour test flight from Filton, England to Toulouse, France in February, 2008 (Vandore). The tests were a combined effort between Qatar Airways, Rolls Royce, and Shell International petroleum. Stephan Vella, a management advisor at Qatar Airways, said his airline hopes to start making modest use of GTL next year (Vandore). GTL may have the most promising potential of alternative jet fuels. The FT industry appears to be on the verge of a period of expansion. Several major companies have announced plans to build large plants which may, if completed, yield approximately 1 million barrels per day by 2020 (Chevron).

Biomass to Liquid

Utilizing the FT process, biomass is converted to an alternate fuel in what is known as the biomass to liquid (BTL) process. Organic materials such as corn stalks, straw, wood chips and waste paper are feed stock for the BTL FT process. Switchgrass, which is a hardy plant, grown in harsh environments shows great potential as a source for feedstock for the BTL process. The benefit of these starter materials being a renewable resource is also a benefit which lends to the continued research in this area. The feedstock is processed in the same gasification manner as the CTL and GTL synthesis and produces essentially the identical product as all connection to the starting material is lost in the synthesis process. The BTL process is among the Fischer-Tropsch synthesis procedures that are becoming reenergized with the U.S. Government’s Total Energy Development (TED) program (NASA). One major concern is the quantity of crops required for BTL synthesis as many countries are unable to produce sufficient fuel feedstock while avoiding deficiencies in agricultural demands.

Biofuel

The use of biomass has had an increase in consideration of late as an alternate fuel for transportation. Ethanol, derived from fermenting sugars from sources such as corn or sugarcane and biodiesel, derived from various vegetable oils and animal fats, have been used successfully in recent years as a blend to existing fuels. Although limited success has been achieved, much research and refinement is critical to full implementation as an alternate fuel in the aviation industry. Government mandates such as the Federal Aviation Administration’s Commercial Aviation Alternative Fuels Initiative (CAAFI), provide a forum for the U.S. commercial aviation community to engage the emerging alternative fuels industry and to work together, share and collect needed data, and motivate and direct research on aviation alternative fuels (FAA).

Ethanol currently is produced primarily by fermenting the agricultural crops corn and sugarcane. Some researchers argue that ethanol production has a negative energy balance (NASA) in that the energy used to produce the fuel outweighs the energy derived from the production. Other concerns are the amount of food crops required for production of energy relative to sustainable edible food crops. One alternative to corn or sugarcane for ethanol production is the use of Switchgrass. It is estimated that 1 acre of Switchgrass could viably produce 500 gallons of ethanol compared to 1 acre of soybeans producing only 50 gallons of bio-diesel (NASA). Cellulose based feedstocks can not be fermented as sugar based crops like corn so, a different conversion process would need to be developed (Chevron). Also the properties of ethanol; gravimetric energy content, volatility, and boiling range, suggest that ethanol may be more practical as a ground transportation fuel but, research continues to determine the viability of ethanol as an alternative for aviation use.

Biodiesel is also under consideration as an alternative to petroleum based jet fuel. The term, biodiesel covers a variety of materials made from vegetable oils or animal fats (Chevron). A major concern with biodiesel, as with ethanol, is sustainability. The primary crop used to produce biodiesel in the U.S. is soybeans. In other countries crops such as, rapeseed in Europe or palm and coconut in Asia are utilized for the oil content of the plants. The concern is the required arable land needed to produce adequate crops to meet the energy demands of the respective country. For example, Europe’s favorite bio-diesel feedstock is rapeseed; to replace only the diesel fuel demand of Germany with bio-diesel would require four times the arable land area currently used for farming and the replacement of every current crop with rapeseed. The resulting shortfall in food production would become a crucial issue (NASA). In the U.S., a 15 percent blend of bio-jet fuel for the commercial airline fleet would require over 2 billion gallons of bio-fuel. Considering soybeans as the feedstock, at 60 gallons of biofuel per acre, an area of arable land approximately the size of Florida would be required (NASA). Computing these figures gives credence to the non-sustainability argument of bio-fuel and has researchers looking into other sources such as algae for biodiesel production.

Cryogenic Fuels

The use of cryogenic fuels in aviation may be possible but, due to research in aircraft design and storage it would be several decades into the future. Cryogenic fuels like Liquid hydrogen and liquid methane are gases that have been cooled to their boiling point and stored as low temperature liquids. Hydrogen from renewable resources is positioned as the fuel of the future (Chevron). However, economical and efficient processes to generate hydrogen from sources such as water or biomass will need to be in place before it can be considered a viable replacement of fossil fuels. There will also be the need for re-engineering aircraft to store the cryogenic fuels and to operate efficiently and ensure safety.

Aircraft Design Challenges

Synthetic jet fuel has basically the same weight, volume, and performance characteristics of petroleum based jet fuel as do bio-jet fuels (NASA). There would not be any substantial design changes to an aircraft to allow the blend or eventual full use of these fuels as alternatives. There are however, design concerns for the use of ethanol and cryogenic fuels in regard to fuel storage on the aircraft. Engine performance changes will need to be addressed for differing fuels, as will the properties of the fuels as it pertains to Elastomers and aromatics. Also, before any alternate fuel is used in place of the fuel originally approved for the aircraft and engine, a thorough approval process must be conformed to ensure specifications for jet fuels are met.

For ethanol to be used as an alternate jet fuel, the aircraft would need to be specifically designed for the fuel. Blending ethanol with petroleum based jet fuel would pose several problems. The physical and chemical properties of the two fuels are different enough to limit the potential of a blend. Ethanol can be mixed with water where water in conventional jet fuel does not mix but could cause significant problems. The 100 percent use of ethanol as an aviation fuel would require a separate storage and distribution system from conventional jet fuel (Chevron). An aircraft designed to be powered by ethanol would require 64 percent more storage volume for the same amount of energy as conventional fuel (NASA). The increase in volume would necessitate a larger wing increasing the empty weight of the aircraft. As ethanol is heavier than conventional jet fuel and has less energy requiring more fuel, the aircraft would see an increase of 35 percent more weight than an aircraft powered by conventional jet fuel. This increase in weight would require the aircraft to be powered by an engine with 50 percent more thrust (NASA). An ethanol powered aircraft would see significant efficiency penalties and are considered unattractive for airline use by today’s standards.

Cryogenic fuels, primarily liquid hydrogen (LH2), are also under consideration as alternative jet fuels, more for the future than immediate present. Because LH2 must be used in its liquid form, aircraft would require design changes as well. Due to pressurization and the need for insulation, LH2 fuel would not be suitable for wing storage on an aircraft. Storage tanks for LH2 would be large and heavy adding to the empty weight of the aircraft. However, the relative lighter weight of LH2 as compared to conventional jet fuel would permit an approximate 5 percent reduction in takeoff weight (NASA). The reduction in takeoff weight would allow for smaller engines with less thrust leading to possible fuel efficiency improvements especially on longer flights. Never the less, the heavier fuel tank penalty, combined with refueling issues revolving around saturation pressure equilibrium resulting in longer turn times, make the Liquefied gas fueled aircraft less than attractive for aircraft use in today’s environment (NASA).

Jet Engine Performance

At present time and for the near future, synthetic fuels manufactured from coal and natural gas appear to be the best candidates for alternative fuel applications. These synthetic fuels do not require any modifications to aircraft or aircraft engines. As previously reported, The United States Air Force (USAF) has successfully powered two of the eight engines of a B-52 Stratofortress and an F-15E Strike Eagle with a 50-50 blend of JP-8 and natural gas-based synthetic fuel. In another test performed by the USAF, a TF33 PW-103 engine burning the 50-50 blend underwent engine emission tests at Tinker Air Force Base, Oklahoma.

The research team measured emissions over a wide range of engine power conditions, from idle to maximum power. For all engine conditions, results indicated that compared to the JP-8 fuel, the F-T fuel blend yields significant reductions in particulate emissions. Specifically, the team observed a ~20%-40% reduction in particle concentration and smoke number and a ~30%-60% reduction in particulate mass. Furthermore, the F-T blend appeared to have negligible effects on most gaseous products, suggesting that it had no adverse effects on the TF33 engine emissions (Propulsion Directorate)

Other benefits of synthetic fuel to jet engines are: superior thermal stability and excellent low-temperature properties. A further potential use may be the ability to reduce the cooling air temperature for turbine blades and reduce engine oil temperatures to improve engine durability (NASA).

In order to use LH2 as an alternate jet fuel, many modifications would have to be accomplished to the engine. The combustor chamber, fuel pumps. supply lines, and control valves would require some modification. There would also need to be a heat exchanger for heating and vaporizing the cryogenic fuel. Though there are benefits, and the use of LH2 in modern aircraft engines is feasible, much technological development is needed (NASA).

Aviation Fuel Approval Process

Aircraft engine and airframe manufacturers ultimately determine the fuel properties required to ensure safe and reliable operation of their equipment. These requirements are embodied in a fuel specification that is part of an aircraft’s type certificate (Chevron). In the U.S., specification ASTM D1655 Jet A/Jet A-1 is used. The jet fuel specifications are at present, based on experience with conventional jet fuels. Some of the characteristics considered are: aromatic content, boiling range, dielectric constant, thermal conductivity, specific heat, surface tension, thermal and storage stability among others. When alternative fuels are considered, the fuel must meet the specification properties, which were developed around petroleum-derived fuels and must meet the ultimate requirement- a fit-for-purpose aviation fuel (Chevron). As previously reported, the only alternative fuel approved for aviation is Sasol’s synthetic CTL jet fuel. Sasol synthetic fuel had been approved as a 50-50 blend with conventional jet fuel since 1999 and has since April, 2008 been approved for 100% synthetic CTL jet fuel for its synthetic fuel produced in Secunda, South Africa. Sasol is currently in the process of sanctioning their synthetic GTL fuels in Qatar and Nigeria and potential CTL ventures in the U.S., China and India (Next Energy). The experience gained in synthetic fuel use has given a level of confidence to engine and airframe manufacturers. The industry is incorporating a generic approval process for future synthetic fuels that will be used as a blend component with conventional fuels up to 50%. Any other alternative fuel would have to go through a full review before approval (Chevron).

Environmental Concerns

The effects on the environment when producing and burning alternative fuels must be considered. Government agencies, the general public, and the industry itself will only accept products that improve on environmental concerns. Still decades away, the use of liquid hydrogen LH2) as an alternative would leave the smallest carbon footprint as it burns cleanly and produces only water vapor as a bi-product. The effects of large quantities of water vapor being emitted at cruise altitude would need to be understood before fully transitioning to hydrogen as an alternative.

Carbon Dioxide (CO2) emissions are a major environmental concern with any alternate fuel being considered currently. With the exception of LH2, bio-jet fuel has the lowest level of CO2 emissions. In fact, CO2 emissions of bio-jet are lower than conventional jet fuel and are the only currently developed alternate fuel able to make that claim (NASA). However, with technological advances in FT processed fuels, and advances in CO2 sequestering during production, the CO2 levels of CTL and GTL will have a considerable reduction of CO2 emissions. Significant decrease of exhaust emission particles has been noted when FT fuel blends have been tested (NASA).

Other environmental concerns must also be addressed in the quest for alternative fuels. Available arable land use relative to food crop production has a significant impact on bio-mass feedstock. The use of fertilizers and available water for irrigation and the disposal of waste products require attention. Also, the effects of mining, water use, and run-off from mining sites and the waste material of mining are issues of concern that will require environmentally sound solutions. In processing raw materials to viable fuel sources, much energy is required. The results are an increase in CO2 emissions adding to environmental concerns. However, CO2 sequestering combined with the benefits of growing plants for biomass which removes CO2 from the atmosphere could potentially have positive results in decreasing CO2 emissions.

Conclusion

Growing concern for energy independence and security along with environmental concerns give credible cause for the increase in searching for alternative energy sources. But, these alternate fuels must have a positive impact on the environment and cannot risk the safe and reliable operation of the systems they are intended to fuel. Since the early days of aviation, researchers have shown viable alternatives to conventional jet fuel. Current conditions have inspired a renewal of this research with promising results for present and future implementation. The FT process of synthetic fuel is practical for aviation use today, while cryogenic fuels require more technological advance. Biofuels may not be best suited for aviation currently but, show much promise in ground transportation. Combining the enhancement of solar power and wind power for stationary energy with increase in biofuels for ground transportation and the use of synthetic fuels for aviation could accomplish the goals of environmentally sound and secure alternative energy sources.

PAUSE is a grassroots group of individuals who have come together to promote safe, sustainable energy and fight for environmental justice. We engage the greater public to stop the fossil fuel industry’s assault on the people of Albany and our environment.